Reapply policies after queue restart (#1737)

### WHAT is this pull request doing?

Reapplies policies after restarting a closed queue. Previously, only
queue arguments (e.g. `x-max-length`) were restored via
`handle_arguments`, but policies set via the management API were not
reapplied.

Fixes #1736 

### HOW can this pull request be tested?
Run the new spec: `make test SPEC=spec/queue_spec.cr`

Author: kickster97

spec/queue_spec.cr

@@ -372,6 +372,29 @@ describe LavinMQ::AMQP::Queue do
       end
     end
 
+    it "should reapply policies after restart" do
+      with_amqp_server do |s|
+        with_channel(s) do |ch|
+          q = ch.queue(q_name, durable: true)
+          queue = s.vhosts["/"].queues[q_name].as(LavinMQ::AMQP::DurableQueue)
+
+          # Apply a max-length policy
+          definition = {"max-length" => JSON::Any.new(2i64)}
+          policy = LavinMQ::Policy.new("test", "/", /.*/, LavinMQ::Policy::Target::Queues, definition, 0i8)
+          queue.apply_policy(policy, nil)
+
+          # Close and restart the queue
+          queue.close
+          queue.closed?.should be_true
+          queue.restart!.should be_true
+
+          # Publish 4 messages, only 2 should be kept
+          4.times { |i| q.publish_confirm "msg #{i}" }
+          queue.message_count.should eq 2
+        end
+      end
+    end
+
     it "should not restart if queue is still running" do
       with_amqp_server do |s|
         with_channel(s) do |ch|

src/lavinmq/amqp/queue/queue.cr

@@ -203,7 +203,9 @@ module LavinMQ::AMQP
       reset_queue_state
       @msg_store = init_msg_store(@data_dir)
       @empty = @msg_store.empty
-      start
+      start.tap do |started|
+        reapply_policy if started
+      end
     end
 
     private def reset_queue_state

src/lavinmq/policy.cr

@@ -7,6 +7,10 @@ module LavinMQ
     getter operator_policy : OperatorPolicy?
     getter effective_policy_args = Array(String).new
 
+    def reapply_policy
+      apply_policy(@policy, @operator_policy)
+    end
+
     def apply_policy(policy : Policy?, operator_policy : OperatorPolicy?)
       clear_policy
       effective_policy_args = Array(String).new